1. Technical Field of the Invention
The present invention relates to novel material compositions comprising copper, tin, an alkali metal and, optionally, zinc and/or phosphorus as catalytic systems especially suited for the direct synthesis of alkylhalosilanes by contacting an alkyl halide with silicon and which makes it possible to conduct this direct synthesis while substantially reducing the formation of coke.
2. Description of Background and/or Related and/or Prior Art
The industrial process for the manufacture of alkylhalosilanes and, for example, of dimethyldichlorosilane, subsequently referred to as DMDCS, is a well known process which is described in particular in U.S. Pat. No. 2,380,995 and in the text by Walter Noll, Chemistry and Technology of Silicones, 1968, published by Academic Press Inc., London, pages 26–41.
According to this “direct synthesis” or “Rochow synthesis” process, the alkylhalosilanes, for example DMDCS, are manufactured directly by reaction of methyl chloride with a solid contact body formed of silicon and of a catalyst comprising copper, according to the reaction:2CH3Cl+Si→(CH3)2SiCl2.
In reality, other coproducts, such as in particular those indicated below, are formed during the direct synthesis: other alkylhalosilanes, such as methyltrichlorosilane CH3SiCl3, subsequently referred to as MTCS, and trimethylchlorosilane (CH3)3SiCl, subsequently referred to as TMCS; halogenated alkylhydrosilanes, such as, for example, methylhydrodichlorosilane (CH3)HSiCl2, subsequently referred to as MHDCS; and heavy products which are polysilanes and in particular disilanes, such as, for example, trimethyltrichlorodisilane (CH3)3Si2Cl3 and dimethyltetrachlorodisilane (CH3)2Si2Cl4.
Among all the products obtained by direct synthesis, the dialkyldihalosilane, and for example DMDCS, is the main product, that is to say the product obtained in predominant amount. This product is highly desirable as, after hydrolysis and polymerization, it makes it possible to obtain oils and gums which are base products for the manufacture of silicones.
It is known to use copper, whether in the form of copper metal or in the form of copper-based chemical compounds, as catalyst of the direct synthesis reaction.
In order, in particular:                to improve the mean activity (also referred to as productivity) of the contact body comprising the combination based on silicon and on catalyst, this activity (or productivity) being evaluated as weight of the silanes obtained per hour and per kilogram of silicon initially involved,        to also improve the selectivity for the dialkyldihalosilane, and for example for DMDCS, evaluated, for example, by the mol % of DMDCS with respect to all the silanes obtained and by the MTCS/DMDCS mean ratio by weight, and        to lower the content by weight of “heavy” products with respect to the silanes obtained,it has to date been proposed to add, to the copper, a promoter combination comprising one or more promoting additive(s). These additives can be: zinc or a zinc halide (U.S. Pat. No. 2,464,033), aluminum (U.S. Pat. Nos. 2,403,370 and 2,427,605), tin, manganese, nickel and silver (GB-A-1-207,466), cobalt (GB-A-907,161), potassium chloride (SU-A-307,650), or arsenic or an arsenic compound (U.S. Pat. No. 4,762,940).        
EP-A-0-138,678 and EP-A-0-138,679 describe the use of a copper catalyst as a mixture with an improved promoter combination which includes:                30 to 1,000 ppm (calculated as weight of metal with respect to the weight of silicon involved) of at least one metal selected from among tin and antimony or of a compound based on tin and/or on antimony,        optionally 0.1 to 3% (calculated as indicated above) of zinc metal or of a zinc-based compound, and        in the case of EP-A-0-1 38,678: 0.05 to 4% (calculated as indicated above) of cesium or of a cesium compound, taken alone or as a mixture with at least one other alkali metal selected from among lithium, sodium, potassium, rubidium and a compound based on said alkali metal; or, in the case of EP-A-0-138,679: 0.05 to 2% (calculated as indicated above) of at least one alkali metal selected from among lithium, sodium, potassium, rubidium and a compound based on said same alkali metal.        
U.S. Pat. No. 4,601,101 describes the use of a copper catalyst as a mixture with another improved promoter combination which includes:                5 to 200 ppm (calculated as weight of metal with respect to the weight of silicon involved) of tin or of a tin-based compound,        optionally 100 to 10,000 ppm (calculated as indicated above) of zinc metal or of a zinc-based compound, and        25 to 931 ppm (calculated as indicated above) of elemental phosphorus, of a metal phosphide and/or of a compound capable of providing a metal phosphide in the reaction body of the direct synthesis.        
However, despite the importance of the catalytic systems (copper catalyst as a mixture with a promoter combination) provided in the abovementioned prior art, research continues in this field as the industrial conditions for implementation of the direct synthesis process give rise, in a manner known per se [cf. Journal of Catalysis, 161, 861–866 (1996)], to side reactions in which the starting alkyl halide is cracked, which side reactions result in the formation of coke and of hydrocarbons. This formation of coke is the cause in particular of the fouling of the direct synthesis reactor(s) within which coke is deposited. This undesirable fouling requires that periodic cleaning operations be carried out, which accordingly reduces the productive capacity of the plant.